The invention relates to a cooling system for cooling a fluid in indirect heat exchange with a cryogenic medium. The cooling system comprises a first passageway (3, 4) having an inlet opening for said cryogenic medium and an outlet opening for said cryogenic medium, and further comprising a medium supply line (6) connected to said inlet opening. A bypass line (5) provides a fluid communication between said outlet opening and said inlet opening.
The invention relates to a cooling system for cooling a fluid in indirect heat exchange with a cryogenic medium comprising a first passageway having an inlet opening for said cryogenic medium and an outlet opening for said cryogenic medium, and further comprising a medium supply line connected to said inlet opening. The invention further relates to a method for cooling a fluid wherein a cryogenic medium is delivered from a cryogenic medium supply and brought into indirect heat exchange with said fluid.
Conventional mechanical cooling systems for cooling goods during transport on a truck use a refrigerant which is compressed and condensed and subsequently expanded. During expansion of the refrigerant the temperature of the refrigerant is greatly reduced and the so produced cold is used to cool down the air within the truck. Such mechanical cooling systems include several movable parts, for example the compressor to compress and circulate the refrigerant, which need high maintenance and which might cause mechanical problems.
There also exist cryogenic cooling systems which use the cold of a cryogenic liquid such as liquid nitrogen or liquid carbon dioxide. The cryogenic liquid is vaporized in indirect heat exchange with the air to be cooled and then vented to the atmosphere.
It is an object of the invention to provide a cooling system and a cooling method which is simple and demands only little maintenance.
This object is achieved by a cooling system for cooling a fluid in indirect heat exchange with a cryogenic medium comprising a first passageway having an inlet opening for said cryogenic medium and an outlet opening for said cryogenic medium, and further comprising a medium supply line connected to said inlet opening, which is characterized by a bypass line providing a fluid communication between said outlet opening and said inlet opening.
The inventive method for cooling a fluid comprises the steps of delivering a cryogenic medium from a cryogenic medium supply, bringing said cryogenic medium into indirect heat exchange with said fluid, and returning at least a part of said cryogenic medium from said outlet opening to said inlet opening.
The invention is preferably used to cool air of a conditioned space, for example to cool the air within a container, a transport truck or a trailer. In the following the invention will be explained with reference to a truck cooling system. It should be understood that the invention as well as the preferred embodiments of the invention is not limited to the described truck cooling system and can also be used for other cooling applications.
The inventive cooling system uses a cryogenic medium to cool the air of the conditioned space or to cool any other fluid. In order to use most of the cooling capacity of the cryogenic medium and thus to increase the efficiency of the cooling system, at least a part of the cryogenic medium is returned from the outlet opening to the inlet opening of the first passageway. The cryogenic medium is again passed through the first passageway and the heat absorbing capacity of the cryogenic medium is used once more.
The first passageway for the cryogenic medium is preferably connected to a venting line. As already mentioned part of the cryogenic medium is returned through said bypass line for re-use. Some of the cryogenic medium is vented through said venting line and normally blown to the atmosphere. The vented cryogenic medium can also be passed to a gas storage container and later be used for other applications, for example for inerting. Preferably the amount of vented cryogenic medium is regulated to keep a small over-pressure of typically 0,1 to 1,0 bar within the first passageway. It is also advantageous to use the vented cryogenic medium to run a fan or another device . In that case the pressure within the first passageway and thus the pressure of the vented cryogenic medium could be higher than 1 bar.
The flow of vented cryogenic medium is preferably controlled by use of a check valve or a regulator in the venting line. Further the venting line might be provided with a muffler.
In a preferred embodiment said first passageway is vertically oriented in order to minimize the pressure drop of the cryogenic medium flowing through the first passageway.
Preferably said medium supply line comprises an expansion nozzle, for example a Laval nozzle. The liquid cryogenic medium is passed through the expansion nozzle and expanded. Thus the temperature of the cryogenic medium can be lowered and the cooling effect can be increased. Further the fast stream of expanded cryogenic medium can be used to suck in cryogenic medium which has been returned through the bypass line.
It is preferred to connect said medium supply line to a liquid cryogen supply, in particular to a liquid carbon dioxide supply or to a liquid nitrogen supply. Liquid carbon dioxide is the most preferred source for the cryogenic medium. The liquid carbon dioxide is sprayed through an expansion nozzle whereas a mixture of dry ice and gaseous carbon dioxide is created. The stream of gas/liquid leaving the nozzle may preferably be used to suck warmer gaseous carbon dioxide from the bypass line which is then mixed with the expanded gaseous carbon dioxide and the dry ice. The relative warm gas from the bypass line supports sublimation of the dry ice which should not be collected in any part of the cooling system.
The liquid carbon dioxide may also be expanded in said expansion nozzle and then used to move or rotate a turbine. The turbine may be designed to act as a fan and to mechanically create a backflow of gaseous carbon dioxide through the bypass line.
It is preferred to have a backflow through said bypass line which is at least 5 times higher, preferably at least 10 times higher than the flow coming out of the nozzle. The nozzle can normally suck approximate 6-7 times the flow out of the nozzle depending on the design of the nozzle and the upper part of the first passageway. In case a higher backflow is needed or desired it is advantageous to use a fan to create and / or regulate the back flow. The typical flow will be approximate 3 litres / s.
Preferably said bypass line is provided with flow regulation means in order to optimize the relation between recycled or returned cryogenic medium and fresh cryogenic medium. The flow through said bypass line is preferably regulated depending on the temperature of the cryogenic medium after it has passed the first passageway, the temperature of the fluid prior to its cooling and / or the temperature of said fluid after it has been cooled. In general it is advantageous to check the temperature in different parts of the cooling system in order to create the right temperature and cooling conditions.
The backflow of cryogenic medium through said bypass line may be caused by the expansion of liquid cryogenic medium which sucks returned cryogenic medium into the first passageway. It is further possible to have a fan or any other means for creating a back flow of cryogenic medium from said outlet opening to said inlet opening.
The fluid to be cooled, in particular air, is normally withdrawn from the space or compartment and then cooled in heat exchange with the cryogenic medium flowing through said first passageway. Finally the cold fluid is returned to said space.
According to a preferred embodiment the cooling system comprises a second passageway being in heat transfering contact with said first passageway. Air from the transport truck or any other fluid which shall be cooled is fed to the second passageway and cooled in indirect heat exchange with the cryogenic medium. Preferably the cryogenic medium and said fluid flow in opposite directions through said first and second passageway, that is in a counter-flow arrangement. It is further preferred that a fan or another mechanical device is used to propel the fluid to be cooled through the second passageway.
From time to time the cooling system has to be defrosted. In a preferred embodiment a defrosting heat exchanger is provided being in heat transfering contact with said second passageway. The easiest way to defrost a cooling system mounted in a transport truck is to take energy from the truck motor cooling system. The heat from the truck motor may be transferred to said defrosting heat exchanger in order to warm the air passed through said second passageway and thus to defrost the second passageway. In order to remove any liquid during the defrosting operation, the second passageway could be provided with a drain valve.
The invention is in particular useful to cool an air-conditioned compartment, especially a storage compartment of a transport truck. In such an application the preferred cryogenic medium is carbon dioxide providing several advantages compared to the
In the known cryogenic cooling systems using liquid carbon dioxide as the heat absorbing medium, the temperature of the air which shall be cooled down is limited by the temperature of the liquid carbon dioxide. According to the invention the temperature of the air can be lowered below the equilibrium temperature of liquid carbon dioxide since during the expansion of the liquid carbon dioxide additional cold is created. The low temperatures that can be reached by the inventive system allow a shorter cooling down time of the parts or the cargo which shall be cooled.
The liquid carbon dioxide used as said cryogenic medium is normally stored in a liquid carbon dioxide tank. Gaseous carbon dioxide from this tank can also be used in the inventive cooling system. If the pressure in the liquid carbon dioxide tank is increased by for example heat losses, the pressure can be reduced by taking gaseous carbon dioxide from the tank, expanding said gaseous carbon dioxide in the same nozzle as the liquid carbon dioxide and passing it through said first passageway.
By regulation of the backflow through the bypass line the temperature of the heat absorbing carbon dioxide can be chosen. The cooling system comprises only few movable parts and thus its maintenance is facilitated. The whole piping and in particular the defrosting system are very simple. The cold air can be distributed to different cooling areas, for example to different parts of the truck, and by using additional temperature sensors and valves it is possible to create different temperature zones by means of one cooling system.
The invention as well as further details and preferred embodiments of the invention are disclosed in the following description and illustrated in the accompanying drawings, in which
Figure 1 shows a cooling system which is used on a transport truck to cool the cargo or to maintain the temperature of the cargo during transportation.
The inventive cooling system utilizes carbon dioxide as heat-absorbing medium. Liquid carbon dioxide is withdrawn from a supply tank (not shown in the drawing) via pipe 6. The flow of liquid carbon dioxide can be regulated by a regulation valve 1. At its downstream end pipe 6 is provided with an expansion nozzle 2 which is preferably vertically mounted. The surrounding of expansion nozzle 2 is designed in such a way to create a maximum flow of gas from bypass line 5.
Valve 1 is so located that when it closes gaseous carbon dioxide will be collected close to valve 1. When valve 1 is opened, this gaseous carbon dioxide will increase the pressure in tube 6 downstream valve 1 and in expansion nozzle 2 so that formation of dry-ice is avoided. In addition it is possible to provide nozzle 2 with a heater (not shown).
Heat exchanger 30 comprises an inner pipe 3, 4 and an annular pipe 31 which is coaxially arranged with inner pipe 3, 4. Of course it is also possible to use a heat exchanger with several pipes. Pipe 6 is connected to the top of inner pipe 3, 4 of heat exchanger 30. The axis of heat exchanger 30 is vertically oriented in order to minimize the pressure drop of carbon dioxide entering inner pipe 3, 4 through pipe 6 and expansion nozzle 2.
The bottom end of inner pipe 3, 4 is interconnected with the top of inner pipe 3, 4 by a bypass line 5. Bypass line 5 is provided with a fan 7 and a regulation valve 8. Further, a venting line 11 is connected to inner pipe 3, 4. Venting line 11 is provided with a check valve 9 or a regulator which opens at a pre-defined overpressure and a muffler 10. The overall design of the upper part 3 of the inner pipe, in particular nozzle 2 and the connection with bypass line 5, is done in such way that the liquid carbon dioxide which is going to the nozzle 2 is partly or completely vaporized before it expands in the nozzle 2.
Air taken from the compartment to be cooled is fed to annular pipe 31 of heat exchanger 30 via line 12. The exact location of the air inlet openings into line 12 is chosen depending on several parameters, for example the size and type of compartment or space to be cooled, the number of different temperature zones within that space, and the type of truck. Fan 18 is used to propel the air through line 12, annular pipe 31 and pipeline 14.
Via pipeline 14 the cooled air is transferred back from outer pipe 31 to the compartment. The location of the air outlet openings of pipeline 14 is also chosen depending on the size and type of space to be cooled. In a preferred embodiment pipeline 14 comprises several air outlet openings which can be regulated individually in order to create different temperature zones. The temperature of the incoming air in line 12 and the temperature of the air leaving heat exchanger 30 can be measured by temperature sensors 13 and 15, respectively.
For defrosting and draining purposes annular pipe 31 is further provided with a drain valve 17 and a heat exchange system 16. In figure 2 heat exchange system 16 is shown in more detail. Heat exchange system 16 comprises two heat exchangers 23 and 32 which are thermally connected by a secondary flow circuit 29. A pump 33 is used to circulate fluid in the secondary flow circuit. Two valves 26, 27 are used in order to regulate the temperature of the secondary flow. The temperature is measured by a temperature sensor 28.
First heat exchanger 23 is thermally connected to the cool/heat system of the truck. Instead of the truck cool/heat system any other heat system could be chosen, but the truck cool/heat system is preferred. A heated liquid, for example a mixture of glycol and water, is passed through line 21 to heat exchanger 23 to warm secondary flow 29. The flow of the glycol/water mixture may be regulated by valves 22, 24.
The operation of the inventive truck cooling system shall be explained in the following.
Valve 1 is closed. The location of valve 1 is so that in its closed status gaseous carbon dioxide from inner pipe 3, 4 will be collected close to valve 1. When valve 1 is opened this gaseous carbon dioxide increases the pressure of the tube volume near nozzle 2 so that dry-ice formation is avoided or at least minimized. It is also possible to provide nozzle 2 with a separate heater (not shown in figure 1) or, in the start procedure, first open a gaseous carbon dioxide valve 34 and then open the liquid valve 1. When liquid valve 1 is open, valve 34 will be closed. Venting gaseous carbon dioxide through valve 34 and nozzle 2 into the cooling system will not only reduce over-pressure in the liquid CO2 supply tank but also reduce the overall cold losses in the whole system.
When valve 1 is opened liquid carbon dioxide sprays through nozzle 2 into the upper part 3 of the inner pipe of heat exchanger 30. During the expansion of the liquid carbon dioxide gaseous carbon dioxide and possibly some dry ice might be formed. The fast stream of expanding carbon dioxide sucks gaseous carbon dioxide from bypass line 5 into the upper part 3 of the inner pipe.
Upper part 3 works as a turbulence chamber where the expanding carbon dioxide from nozzle 2 and warm gaseous carbon dioxide from bypass line 5 are mixed. The warm gaseous carbon dioxide from bypass line 5 supports melting of any dry ice formed during the expansion of the liquid carbon dioxide. In this way it is assured that no dry ice will be collected in any part of the system.
The gaseous carbon dioxide flows downwardly through inner pipe 3, 4 and absorbs heat from the air flowing in countercurrent direction through the annular pipe 31. Thus the carbon dioxide is warmed up and the air is cooled down.
Approximately 4 to 7 times of the liquid carbon dioxide entering the inner pipe 3, 4 through nozzle 2 is taken back as a back stream through bypass line 5. The back stream is sucked by the expanding liquid carbon dioxide, and additionally needed flow of gas is propelled by a fan 7 located in bypass line 5. The flow of the back stream can be regulated by regulation valve 8 or better by regulating the speed of the fan.
In another embodiment the liquid carbon dioxide passing nozzle 2 enters a turbine, not shown in the figures. The turbine is so designed that part of it acts as a fan or that it drives a separate fan which mechanically creates backflow through bypass line 5. The nozzle 2 could then be changed to create best mixing of the two gas streams. There are several ways to rotate a fan by using a stream of gas. For example an impeller with an inner and an outer ring is used. The gas from nozzle 2 enters the inner ring of the impeller in the middle and is by several "channels" turned out from the middle of the impeller into a tangential direction. The outer ring is designed as a fan, which will circulate the flow of gas through bypass line 5 into the upper part 3 of the first passageway. The mixing of cold and warm gas and the dry-ice will be very good.
Some of the gaseous carbon dioxide flowing in inner pipe 3, 4 is vented through check valve 9, muffler 10 and venting line 11 to the atmosphere. The set pressure of check valve 9 is normally chosen to keep a small over-pressure in the system of about 0,1 to 1 bar. It is also possible to keep the over-pressure in the system at a higher value and then to use the gas vented through venting line 11 to run a fan, for example fan 18.
Warm air from the truck compartment, which shall be cooled down or kept at cold temperature, is propelled through annular pipe 31 by fan 18. By indirect heat exchange with the gaseous carbon dioxide flowing in inner pipe 3, 4 the air is cooled down and then passed back to the truck compartment. The cold air coming out from outlet 14 is used to cool one or several parts of the truck to different temperatures. The temperature is checked in different parts (13, 15) of the system in order to create the desired temperature and cooling conditions.
From time to time the system has to be defrosted. The easiest way to achieve this is to take energy from the motor truck cooling system. The heat is either directly transferred to the system at point 16 or a secondary heat transfer system as shown in figure 2 is used. In the later case the secondary heat transfer system 29 contains a liquid which can also handle low temperatures. It is also possible to directly transfer the heat from the truck to heat exchanger 30 by a complex piping.